The current study seeks to examine whether the observed improvements noted in conduction times in central nervous system pathways in the phase I 'Study of Intravenous Autologous Marrow in Multiple Sclerosis (SIAMMS)' persist several years following the initial single infusion and whether these can be either replicated or augmented by an additional infusion of autologous bone marrow cells and analysis of research samples will be performed as per samples included in the concurrent phase 2 clinical trial 'Assessment of bone marrow-derived cellular therapy in progressive multiple sclerosis (ACTiMuS)' (REC 12/SW/0358, ISRCTN27232902, NCT01815632).